Continuous web printing allows economical, high-speed, high-volume print reproduction. In this type of printing, a continuous web, such as print media (e.g., paper) or a support mechanism in which the print media is disposed over, is fed past one or more printing subsystems or modules that form images by applying one or more colorants onto the surface of the print media. Various components within a printing system are used to create tension in the web so that the web does not shift in the in-track (the direction of movement) and cross-track directions as the web moves through the printing system. The tension is also used to inhibit fluttering (up or down motion) as the web travels through the printing system.
Some of the components used to produce and adjust the tension in the web or to transport the web through a printing system are stationary bars placed at different locations within the printing system. Stationary bars include turnbars and S-wraps. Turnbars are frequently used to change the direction of the web or as a pair to flip over the print media, while S-wraps are used to produce tension in the web. The stationary bars have curved surfaces that the web slides over during transport. As the web is pulled over these surfaces, the friction of the web across these surfaces produces tension in the web.
Some bars are “static” in which the web runs directly on the bar. For turnbar applications, a cover material with a low coefficient of friction is typically applied over the contact surface to reduce drag from the web. Generally, this low coefficient of friction cover material is adhered with adhesive directly onto the exterior surface of the bars. The material must be applied smoothly, with no wrinkles or bubbles that could adversely affect web tracking.
Typically, the web travels through the printing system at a high rate of speed. By way of example only, the web can travel six hundred to one thousand feet per minute. Over time, the combination of the high speed and the friction of the web produce heat and wear on the low-friction cover material on the exterior surfaces of the bars. The low-friction cover material can wear to the point of causing web handling problems. Since the cover material is adhered directly to the bars, it can be difficult and time consuming to replace the material. When the wear reaches an unacceptable level, the bars are removed from the printing system and either the low-friction cover material re-conditioned or the bar is replaced entirely. Replacing the bars can take several hours, during which time the printing system is shut down. Moreover, the alignment of the web in the printing system must be re-calibrated once the bars have been replaced. These actions increase the cost of a print job and reduce the throughput of the printing system.